There has been considerable progress in identifying the genes which encode class Ib molecules and in addressing their structure, biosynthesis, expression and genetics. In this application, we build on our previous observations and continue to examine class Ib molecule structure and function. Our long term goal is to understand under what physiological settings the immune system may utilize class Ib molecules in the presentation of self and/or foreign antigens and to evaluate if they play a role in regulating immunopathological processes. The nature of the peptides bound to MHC encoded molecules influences self- tolerance as well as the effectiveness of T cell responses to foreign pathogens. In studies characterizing Qa-l, the protein product of the class Ib gene T23, we observed that expression can be up-regulated by heat shock (stress) and can bind a peptide derived from Mycobacterium Hsp-65. These observations are consistent with Qa-l having a role in the presentation of peptides derived from stress proteins to T cells. This observation is relevant since immune responses to stress proteins are documented to develop during autoimmune disease and in the immune response to pathogens. We have proposed a series of gene transfection and expression studies designed to directly examine the role of stress proteins in regulating Qa-l expression. Studies have also been designed to identify and characterize T cell subsets which recognize peptides derived from heat-shock peptides bound to Qa-l. Conventional class i molecules (H-2K/D/L and HLA-A/B) have been shown to bind to numerous (lOO's) cellular peptides derived from endogenous or exogenous origins. Based on the selective expression patterns as well as the lack of polymorphism, class Ib molecule have been hypothesized to bind a limited set of peptide moieties. Studies described in this application will extract peptides from class Ib molecules such as Qa-l, and using alloreactive CTLs and sensitive structural analysis will directly address this hypothesis. By examining the diversity of peptides bound to class Ib molecules and recognized by alloreactive T cells we will provide insight into their role as minor transplantation antigens. Moreover, knowledge of the overall features of peptides that bind to class Ib molecules will facilitate future studies aimed at identifying potential self and foreign peptides that bind to class Ib molecules for presentation to T cells.